Organic electroluminescent (EL) elements include a thin film containing a luminescent organic compound interposed between a cathode and an anode. Electrons and holes are injected into a thin film where they are recombined to create excitons. Light is emitted by utilizing luminescence (phosphorescence or fluorescence) upon deactivation of excitons. The organic EL elements are characterized by plane light emission at a high luminance of about 1000 cd/m.sup.2 with a low voltage of less than 10 volts and light emission in a spectrum from blue to red color by a simple choice of the type of fluorescent material.
The organic EL elements, however, are undesirably short in effective life, less durable and less reliable because of the following factors.
(1) Physical changes of organic compounds: To prevent dielectric breakdown of elements due to defects along grain boundaries and surface non-uniformity, an organic compound capable of forming a smooth amorphous thin film must be used. However, the film is less stable in quality and thus allows crystallization to take place with the lapse of time and such crystallization is promoted by a temperature rise of the element due to heat build-up during operation, resulting in a change of film quality. There is a need for a material which can be made vitreous by simple means such as vacuum evaporation and maintains the vitreous state in a stable manner.
(2) Oxidation and stripping of the cathode: Metals having a low work function such as Mg, Li and Na are used as the cathode in order to increase its electron injection efficiency while these metals are reactive with oxygen and moisture in air. As a result, the cathode can be stripped from the organic layer.
(3) Low emission efficacy and increased heat build-up: As the element temperature rises, crystallization is promoted and the organic layer can be melted and broken. It is desired to increase the conversion efficiency of input energy and to find a material capable of imparting heat resistance to the organic layer.
(4) Opto-chemical and electro-chemical changes of organic compounds.
Shirota who is one of the present inventors has made a number of proposals and reports about .pi.-electron system amorphous molecular materials as disclosed in Japanese Patent Application Kokai (JP-A) No. 308688/1992, Polymer Preprints, Japan, Vol. 41, No. 3 (1992), the Japanese Chemical Society's 61st Spring Meeting in 1991, 3D3 36, 3D3 37, 3D3 38, 4F8 18, 4F8 25, 4F8 26, 4F8 32, and the Japanese Chemical Society's 63rd Spring Meeting in 1992, P2571-2574.